Removal of platforms at sea

ABSTRACT

The present invention relates tot a method of removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the method comprising the subsequent steps of:
         providing a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above;   providing a support structure which is configured to rest on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that are constructed to extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform;   positioning the vessel between the legs of the platform;   connecting the connection ends to the legs of the platform;   disconnecting the upper part of the platform from the lower part;   lifting the upper part of the platform from the lower part by the vessel via the support structure;   moving the vessel and the upper part of the platform away from the lower part of the platform.       

     The present invention further relates to a system for carrying out the method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/102,570, filed Dec. 10, 2007, the contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method and system for removing a part of a platform at sea, the platform resting on a seabed.

Methods for removing such platforms are known in the field of the art.

It is widely known that platforms at sea, for instance in the oil and gas industry, have a certain life span, after which they are to be decommissioned and removed. This removal process often is a costly, complicated and sometimes dangerous operation. There is a general need in the field of the art for cost-effective, simple, and safe methods for removing such platforms.

PRIOR ART

US2003/0108392 discloses a system for removing decks from offshore structures. A drawback of this system is that it requires two barges, which is expensive and cumbersome during transport.

GB2165188 discloses a method for installing and removing an upper part of a platform (called deck module) at sea from a leg assembly of the platform.

A disadvantage of the system of GB2165188 is that it comprises a support frame 20 (comprising of girders 51) which is integral with the deck module itself, see FIG. 2 and page 1, lines 78-80 of GB2165188. The support frame 20 is essential to the system, and hence the system of GB2165188 can only by used for removing deck modules having such a support frame 20. Since most existing platforms do not have such a support frame 20, the value of the system of GB2165188 is limited in practice.

A disadvantage of the system of GB2165188 is further that it requires barges which extend to a substantial height above the water surface, in order to remove the deck module from the leg assembly. This is due to the fact that for safety reasons, the deck module is located at a substantial height above the water line, and consequently the support structure 20 is also located at a substantial height above the water line.

WO98/24980 discloses a system for lifting a sea-going structure. One drawback of this system is that it requires two vessels, which is expensive and cumbersome during transport. Another drawback of this system is that it is difficult to unload the sea-going structure from the vessels once it is lifted with the vessels.

OBJECTS OF THE INVENTION

It is an object of the invention to provide an alternative method and device for removing an upper part of a platform which rests on a seabed.

It is an object of the present invention to provide a method and device suffering less from at least one of the drawbacks mentioned above.

It is an object of the invention to provide a simple, cost-effective and reliable method and device for removing an upper part of a platform which rests on a seabed.

At least one of the objects is achieved in a method of removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the method comprising the subsequent steps of:

providing a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above;

providing a support structure which is configured to rest on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that are connection ends configured to be connected to the legs of the platform;

positioning the vessel between the legs of the platform;

connecting the connection ends to the legs of the platform;

disconnecting the upper part of the platform from the lower part;

lifting the upper part of the platform from the lower part by the vessel via the support structure;

moving the vessel and the upper part of the platform away from the lower part of the platform.

The method is simple because the support structure is connected directly to the upper part of the platform. The connection may be effected by welding the support structure to the platform. Other types of connections such as bolting or using special coupling members are also possible.

The method can be dedicated to removing platforms, and thus can be made free of any requirements or equipment relating to installing such a platform. This further specialization allows a simplification of the system, in contrast with complex systems of the prior art.

The method may also be used to remove the lower part of the platform or to remove the entire platform at once.

Generally, the support structure is not fixed to the vessel prior to the moving of the vessel to a position at the platform. Once the vessel is in position, the support structure is handed over to the platform in order to eliminate movement of the barge caused by waves and or wind. This can be achieved for instance by laying the support structure into supports connected to the piles before the vessel arrived, or by means of winch wires lowered from the platform deck. The vessel is then ballasted down so that the support beams come free of the barge and can be welded to the platform legs without interference from movement of the vessel during the welding.

After connection of the support beams, the vessel is positioned and de-ballasted until the barge deck makes contact again with the support beams. The vessel is de-ballasted to an extent that the barge presses onto the support beams and thereby movement of the barge is minimized. Then the support beams are welded to the vessel deck.

The moment of contact between the vessel and the support beams is critical. Measures may be taken in order to minimize the impact between the structures. For instance shock absorbers may be used. Alternatively a hydraulic system may be used, that can pull the vessel towards the support beam.

Generally, positioning the vessel between the legs of the platform is performed prior to connecting the connection ends to the legs of the platform. In this case, it is the vessel which has an important function in positioning the support beams. However, it is also possible that the beams of the support structure are connected to the legs of the platform when the vessel is not present. The vessel may pre-position the support beams and be removed during the connecting of the connection ends to the legs. Alternatively, the vessel may not be involved at all in the connecting of the beams to the legs of the platform. In this case, the support beams may be positioned by a crane on the platform itself and the vessel is positioned between the legs of the vessel only after the support structure is connected to the legs.

Generally, the support structure rests on the deck of the vessel and extends substantially horizontally.

The hull of the vessel will generally be elongate and can also be referred to as a mono-hull.

In an advantageous embodiment, the support structure comprises a hydraulic system for raising the one or more beams including the platform relative to the vessel, the method comprising raising the upper part of the platform relative to the vessel by the hydraulic system while at the same time deballasting the vessel. In this way, the duration of the time period in which the upper part and the lower part of the platform are so close together that they are at risk of collision stays relatively short.

In an aspect of the invention, the method comprises:

positioning the one or more beams of the support structure in a target position wherein the connection ends engage the legs;

supporting the one or more beams in their target positions by the platform via provisional or preliminary support means on the platform such that possible motions of the support vessel under influence of waves and wind do not affect or disturb the connecting of the one or more beams to the legs;

connecting the one ore more beams to the legs.

Advantageously, the connecting of the support structure to the legs can take place without hindrance from movement of the vessel.

The provisional or preliminary support means may be a ridge, collar or other projection on the legs of the platform on which the support structure is placed and comes to rest.

In an advantageous embodiment, the method comprises:

positioning the one or more beams of the support structure in a target position wherein the connection ends engage the legs;

supporting the one or more beams in their target positions by the platform via provisional or preliminary support means on the platform;

disconnecting the support structure from the vessel such that possible motions of the support vessel under influence of waves and wind do not affect or disturb the connecting of the one or more beams to the legs;

connecting the one ore more beams to the legs;

reconnecting the support structure to the vessel prior to the lifting of the upper part of the platform from the lower part of the platform.

When the vessel is disconnected from the support structure, the connecting operation can be performed under relatively quiet circumstances. To this end, the vessel may be ballasted to a deeper draft or a hydraulic lift system may be operated to lower the support structure relative to the deck of the vessel, thereby gradually transferring the weight of the beams to the legs of the platform.

In one aspect of the invention the support structure comprises one or more rails or skid tracks which rest on the vessel, wherein the beams of the support structure are movably arranged on the one or more rails or skid tracks via wheels or skid means, the method comprising rolling or skidding the one or more beams relative to the vessel for unloading the upper part of the platform from the vessel.

This method allows very heavy platforms to be unloaded fro the vessel with relative ease.

In another aspect, the method comprises removing the upper part of the platform from the vessel by

positioning one or more support carriages on the deck of the vessel,

disconnecting the one or more beams from either the vessel deck or the platform

lifting the upper part of the platform optionally including one or more beams from the deck

unloading the combination of support carriage, beams and upper part of the platform from the vessel.

Via the support carriages, the upper part of the platform is simply rolled off the vessel.

The present invention further relates to a system for removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the system comprising:

a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above;

a support structure which is configured to rest on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that are constructed to extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform.

The present invention also relates to a support structure (i) for use in the method of the present invention of removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the method comprising the subsequent steps of: providing a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above; providing a support structure which is configured to rest on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that are constructed to extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform; positioning the vessel between the legs of the platform; connecting the connection ends to the legs of the platform; disconnecting the upper part of the platform from the lower part; lifting the upper part of the platform from the lower part by the vessel via the support structure; and moving the vessel and the upper part of the platform away from the lower part of the platform; or (ii) for use in the system of the present invention for removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the system comprising: a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above; and a support structure which rests on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform.

The present invention further relates to a vessel (i) for use in the method of the present invention of removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the method comprising the subsequent steps of: providing a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above; providing a support structure which is configured to rest on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that are constructed to extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform; positioning the vessel between the legs of the platform; connecting the connection ends to the legs of the platform; disconnecting the upper part of the platform from the lower part; lifting the upper part of the platform from the lower part by the vessel via the support structure; and moving the vessel and the upper part of the platform away from the lower part of the platform; or (ii) for use in the system of the present invention for removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the system comprising: a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above; and a support structure which rests on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform.

The present invention further relates to a docking station for unloading the platform from the vessel, the docking station comprising a bay for the vessel and support members positioned around the bay, the support members being configured to support the support beams of the support structure, thereby allowing the unloading method of:

positioning the vessel with the upper part of the platform in the docking station,

ballasting the vessel such that the vessel moves downward and the support beams engage the support members, thereby shifting the weight of the upper part of the platform from the vessel to the docking station via the one or more support beams;

removing the vessel from the docking station.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a platform removal system according to the prior art;

FIG. 2 shows an isometric view of an embodiment of the invention;

FIG. 3 shows a front view of a platform removal system of the invention supporting an upper part of the platform;

FIG. 4 shows a side view of a platform removal system of the invention carrying an upper part of the platform;

FIGS. 5 a-5 g show different steps in a platform removal method according to the invention; and

FIG. 6 shows an isometric view of an alternative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 a and 1 b show the system of GB2165188 and disclose a vessel 70. The platform (or jacket structure) is built with a specific future removal method in mind. To this end, the deck module 16 of the platform is provided with a horizontal beam 51. Further, support members 76 a are provided at the underside of the beam 51 via which the deck module 16 is removably supported by the leg construction of the platform via support points 24 a. This combination of features allows the removal vessel 70 to move between the legs of the jacket. Lifting means 56 are provided which engage with outwardly projecting ends of the beams 51. The deck module 16 is subsequently lifted integrally from the support positions 24 a.

As mentioned, the method and system of GB2165188 can only be used for platforms which are specifically designed and constructed to be removed with this system. If the platform does not have a beam 51 with outwardly projecting ends or if the platform does not have support points 24 which allow a disconnection of the deck module from the jacket legs, the system of GB2165188 does not work or works only at high costs. See page 1, lines 78-80 of GB2165188 which disclose that the support frame is integral with the deck module. Since most existing platforms do not have such a support frame, the value of the system of GB2165188 is limited in practice.

A further disadvantage of the system of GB2165188 is that the deck module including the beams 51 generally is located at a substantial distance above the mean sea level water line in order to stay clear of storm waves. This distance is generally a design requirement for safety purposes of platforms. In that case, the lifting means 56 also need to be provided high above the water line, which is complicated and disadvantageous.

In particular the strut assemblies 50 shown in FIGS. 1 a and 1 b may need a substantial length in order to reach the girders 51.

FIGS. 2, 3 and 4 show an embodiment of the platform removal system according to the invention. A vessel or barge 2 is provided having skid means 33 on the deck 46. The barge 2 is substantially elongate and the skid means 33 extend in the longitudinal direction of the barge 2. The skid means 33 may be rails or tracks or simply a path defined on deck 46. Alternatively or additionally, deck supports 33 may be provided.

The jacket structure or platform 1 which is to be removed rests on the seabed (not shown) on legs 5. The platform 1 comprises a top module or deck module 6 which is located above the water line 8. The barge 2 has a width 62 which is smaller than the distance 64 between two legs 5 of the platform 1, allowing the barge 2 to be positioned between the legs 5 of the platform 1.

The removal system further comprises a support structure 30 comprising one or more beams 31. The beams 31 have ends 32 which are connected to the legs 5 of the platform. The beams 31 are configured to rest on the skid means 33 and are supported in such a way that they can move relative to the barge 2 by sliding or rolling.

The vertical distance between the beam 31 and the underside 44 of the deck module 6 may be substantial. The beam 31 may be located substantially below a nominal wave height, whereas the deck 42 and in particular the underside 44 of the deck module 6 is located at a substantial height above the nominal wave height Hs. This feature allows the platform removal system to be relatively low.

In FIG. 3 a front view of the removal system is shown, the barge 2 carrying the upper part of the platform. The beams 31 are connected to the legs 5 at connection points 32.

FIG. 4 shows the system from the side. The skid means 33 may extend beyond the hull of the barge 2 for easy roll-off or skid-off of the support structure 3 with the upper part of the platform from the barge 2.

FIG. 5 a shows a first step in a method for removing the upper part 11 of a platform 1. Hang-off provisions or support members 4 are connected to the legs 5. The support members 4 may be steel beams or collars which are welded to the legs 5 of the platform 1. This step may be executed prior to the arrival of the barge 2.

FIG. 5 b shows a next step wherein the barge 2 arrives with the beams 31 supported on the deck 46. The beams 31 may be uncoupled from the barge 2. The barge 2 is positioned such that the beams 31 come to rest on the support members 4.

Subsequently, the barge 2 is ballasted by filling ballast compartments in order to lower the barge 2 relative to the beams 31. The beams 31 come loose from the barge 2 and are now fully supported by the support members 4. The beams 31 can now be fixed to the legs 5.

It is also possible that the beams 31 are lifted relative to the barge 2 and placed on support members 4. In an alternative variant, the barge 2 may not be present during installation of the beams 31 on the support members 4. In this case, a crane vessel installs the beams 31, or the beams may be installed by the workers on the platform 1 itself, without the use of an additional vessel.

In FIG. 5 c, it is shown that in a next step the ends 32 of the beams 31 are connected to the legs 5 of the platform 1. Welding is a suitable way of making the connection.

FIG. 5 d shows that in a next step, the barge 2 is positioned such that the beams 31 engage the skid means 33 on the deck 46. Generally, the barge 2 will be deballasted by emptying the ballast compartments 7. However, it is also possible that connection means extend upwards from the deck 46 of the barge 2 and form a connection between the deck 46 and the beams 31.

FIG. 5 e shows that in a next step, the legs 5 of the platform are cut, such that the platform 1 is divided in an upper part 11 and a lower part 12. Although the drawing shows that the legs are cut above the water line, alternatively this can be done beneath the water line.

FIG. 5 f shows how in a next step, the barge 22 lifts the upper part 11 from the lower part 12 via the beams 31. This may be performed by further deballasting the barge 2. In order to accelerate this step, a further hydraulic lifting system 34 may be provided on deck 46, which may be operated simultaneously with the deballasting. The hydraulic lifting system 34 is configured to move the beams 31 upward relative to the deck and to lower the beams 31 relative to the deck 46.

FIG. 5 g shows a situation in which the upper part 2 is completely removed and the lower part 12 remains behind. The lower part 12 may be removed separately.

FIG. 6 shows an alternative embodiment, in which carriages 80 are used to unload the upper part 11 of the platform from the barge 2. The upper part may be positioned on the carriages directly after removal from the lower part of the platform, but it is also possible that the upper part is placed on the carriages when the barge arrives at the unloading location. Hydraulic lifting means 34 may be present for lifting the upper part 11 from the deck 46 and onto the carriages 80.

It will be obvious to a person skilled in the art that numerous changes in the details and the arrangement of the parts may be varied over considerable range without departing from the spirit of the invention and the scope of the claims. 

1. A method of removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the method comprising the subsequent steps of: providing a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above; providing a support structure which is configured to rest on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that are constructed to extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform; positioning the vessel between the legs of the platform; connecting the connection ends to the legs of the platform; disconnecting the upper part of the platform from the lower part; lifting the upper part of the platform from the lower part by the vessel via the support structure; and moving the vessel and the upper part of the platform away from the lower part of the platform.
 2. The method of claim 1, the vessel further comprising a ballast system for varying the draft of the vessel, the method comprising: ballasting the vessel to a deep draft when positioned between the legs of the vessel; and lifting the upper part of the platform by deballasting the vessel.
 3. The method of claim 1, wherein the support structure comprises a hydraulic system for raising the one or more beams including the platform relative to the vessel, the method comprising raising the upper part of the platform relative to the vessel by the hydraulic system, optionally while at the same time deballasting the vessel.
 4. The method of claim 1, comprising pre-assembling the support structure prior to the removal operation in order to fit the support structure to the size and shape of the platform which is to be removed.
 5. The method of claim 1, comprising: positioning the one or more beams of the support structure in a target position wherein the connection ends engage the legs; supporting the one or more beams in their target positions by the platform via provisional or preliminary support means on the platform such that possible motions of the support vessel under influence of waves and wind do not affect or disturb the connecting of the one or more beams to the legs; and connecting the one ore more beams to the legs.
 6. The method of claim 1, comprising: positioning the one or more beams of the support structure in a target position wherein the connection ends engage the legs; supporting the one or more beams in their target positions by the platform via provisional or preliminary support means on the platform; disconnecting the support structure from the vessel such that possible motions of the support vessel under influence of waves and wind do not affect or disturb the connecting of the one or more beams to the legs; connecting the one ore more beams to the legs; and reconnecting the support structure to the vessel prior to the lifting of the upper part of the platform from the lower part of the platform.
 7. The method of claim 1, wherein the beams are releasably arranged on the vessel such that the upper part of the platform and the support structure connected to it can be unloaded from the vessel by moving the beams relative to the vessel, the method comprising: moving the combination of the vessel and the upper part of the platform to a target location, and unloading the upper part of the platform from the vessel by moving the beams and the upper part away from the vessel.
 8. The method of claim 1, comprising unloading the upper part of the platform from the vessel by moving the one or more beams including the upper part of the support structure substantially horizontally relative to the vessel.
 9. The method of claim 3, wherein the support structure comprises one or more rails or skid tracks which rest on the vessel, wherein the beams of the support structure are movably arranged on the one or more rails or skid tracks via wheels or skid means, the method comprising rolling or skidding the one or more beams relative to the vessel for unloading the upper part of the platform from the vessel.
 10. The method of claim 1, comprising removing the upper part of the platform from the vessel by positioning one or more support carriages on the deck of the vessel; disconnecting the one or more beams from either the vessel deck or the platform; lifting the upper part of the platform optionally including one or more beams from the deck; and unloading the combination of support carriage, beams and upper part of the platform from the vessel.
 11. The method of claim 1, comprising unloading the upper part of the platform by moving the support structure and the upper part of the platform in a longitudinal direction of the vessel.
 12. The method of claim 1, comprising unloading the upper part of the platform by moving the upper part of the platform in a transversal direction of the vessel.
 13. The method of claim 1, comprising unloading the upper part of the platform by providing a docking station comprising a bay for the vessel and supports member positioned around the bay, the support members being configured to support the support beams of the support structure, the method comprising: positioning the vessel with the upper part of the platform in the docking station; ballasting the vessel such that the vessel moves downward and the support beams engage the support members, thereby shifting the weight of the upper part of the platform from the vessel to the docking station; and removing the vessel from the docking station.
 14. A system for removing an upper part of a platform which rests on a seabed and which extends above the waterline, the platform comprising legs which extend upwards from the seabed, the system comprising: a vessel comprising a single hull, the hull defining a contour of the vessel when viewed from above; and a support structure which rests on the vessel, the support structure comprising one or more beams, the one or more beams having connection ends that extend outside the contour of the hull on opposite sides of the vessel, the connection ends configured to be connected to the legs of the platform.
 15. The system of claim 14, the vessel further comprising a ballast system for varying the draft of the vessel.
 16. The system of claim 14, wherein the one or more beams are movable in a substantially horizontal direction relative to the vessel.
 17. The system of claim 14, wherein the support structure comprises a hydraulic system which rests on the vessel and is configured to raise the one or more beams including the platform relative to the vessel.
 18. The system of claim 14, wherein the support structure comprises one or more beams having connection ends which project substantially horizontally away from the vessel, a central part of the beam resting on the vessel, such that the connection ends meet the legs of the platform for connection with the legs when the vessel is positioned between the legs of the upper part for the platform.
 19. The system of claim 14, wherein the connection ends of the support structure extend on opposite sides of the vessel, one end of a first beam being configured to be connected to a first leg of the platform, and the opposite end of the first beam being configured to be connected to a second leg of the platform.
 20. The system of claim 14, wherein the one or more beams are releasably arranged on the vessel such that the one or more beams are unloadable at a target location from the vessel together with the upper part of the platform.
 21. The system of claim 14, wherein the support structure comprises one or more rails or skid tracks which rest on the vessel, wherein the beams of the support structure are movably arranged on the one or more rails or skid tracks via wheels or skid means, such that the one or more beams may be rolled or skidded in a substantially horizontal direction relative to the vessel for unloading the one or more beams and the upper part of the platform from the vessel.
 22. The system of claim 14, wherein the support structure comprises one or more support carriages which rest on the deck of the vessel, the support carriages configured to carry the beams including the upper part of the platform, the support carriages further configured to move substantially horizontally relative to the deck for unloading the upper part of the platform from the vessel.
 23. The system of claim 14, wherein the support structure comprises one or more rails or skid tracks which rest on the vessel, and wherein the rails or skid tracks extend in a longitudinal direction of the vessel.
 24. The system of claim 14, further comprising provisional or preliminary support means constructed to support the one or more support beams in a target position on the platform such that possible motions of the support vessel under influence of waves and wind do not affect or disturb the connecting of the one or more beams to the legs.
 25. A support structure for use in the method of claim
 1. 26. A support structure for use in the system of claim
 14. 27. A vessel for use in the method of claim
 1. 28. A vessel for use in the system of claim
 13. 29. A docking station for unloading the platform from the vessel, the docking station comprising a bay for the vessel and support members positioned around the bay, the support members being configured to support the support beams of the support structure, thereby allowing the unloading method of: positioning the vessel with the upper part of the platform in the docking station, ballasting the vessel such that the vessel moves downward and the support beams engage the support members, thereby shifting the weight of the upper part of the platform from the vessel to the docking station via the one or more support beams; and removing the vessel from the docking station. 